Pop Operation on Stack in DSA Python - Execution Trace

Concept Flow - Pop Operation on Stack

Check if stack is empty?

No↓

Remove top element

↓

Update top pointer

↓

Return removed element

↓

Done

Yes↓

Stack Underflow - cannot pop

↓

Done

The pop operation first checks if the stack is empty. If not, it removes the top element, updates the top pointer, and returns the removed element. If empty, it signals underflow.

Execution Sample

DSA Python

stack = [10, 20, 30]
top = 2
popped = stack.pop()
top -= 1
print(stack)

This code removes the top element from the stack and updates the top pointer.

Execution Table

Step	Operation	Stack Before	Top Pointer Before	Element Removed	Top Pointer After	Stack After	Visual State
1	Check if stack is empty	[10, 20, 30]	2	N/A	2	[10, 20, 30]	Stack not empty, proceed to pop
2	Remove top element	[10, 20, 30]	2	30	2	[10, 20]	Element 30 removed from top
3	Update top pointer	[10, 20]	2	30	1	[10, 20]	Top pointer moved down to index 1
4	Return removed element	[10, 20]	1	30	1	[10, 20]	Pop operation complete, returned 30
5	Check if stack is empty (empty case)	[]	-1	N/A	-1	[]	Stack empty, cannot pop, underflow

💡 Pop stops after removing the top element and updating the top pointer, or stops immediately if stack is empty.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
stack	[10, 20, 30]	[10, 20, 30]	[10, 20]	[10, 20]	[10, 20]
top	2	2	2	1	1
popped	N/A	N/A	30	30	30

Key Moments - 3 Insights

Why do we check if the stack is empty before popping?

Because popping from an empty stack causes an error (underflow). The execution_table row 1 shows the check prevents this by stopping if empty.

Why does the top pointer decrease after popping?

Because the top pointer always points to the last element in the stack. After removing the top element (row 2), the pointer moves down by one (row 3) to reflect the new top.

What happens to the removed element after pop?

The removed element is returned by the pop operation (row 4) and no longer part of the stack (stack state changes in row 2).

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'top' after step 3?

A -1

B 2

C 1

D 0

💡 Hint

Check the 'Top Pointer After' column in row 3 of execution_table.

At which step does the stack actually remove the top element?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Operation' and 'Element Removed' columns in execution_table.

If the stack is empty, what happens according to the execution_table?

A Underflow occurs and pop stops

B Pop removes an element

C Top pointer increases

D Stack adds a new element

💡 Hint

Refer to the last row in execution_table describing the empty stack case.

Concept Snapshot

Pop Operation on Stack:
- Check if stack is empty (underflow check)
- Remove top element
- Decrease top pointer by 1
- Return removed element
- Stack shrinks by one element

Full Transcript

The pop operation on a stack first checks if the stack is empty to avoid errors. If not empty, it removes the top element, updates the top pointer to the next element below, and returns the removed element. If the stack is empty, it signals underflow and stops. The top pointer always points to the current top element index. After popping, the stack size decreases by one.